The sensitivity of neuronal development to blockade of electrical activity has an abrupt onset during the seventh day in culture. Cholinergic neurons, but not neurons synthesizing or taking up gramma-amino butyric acid (GABA) are affected. The developmental defects produced by blockade of electrical activity can be prevented by conditioned medium from active (but probably not inactive) cultures. Catechol and indole amine receptors appear to be regulated by amine containing fibers and non-innervated cells exhibit marked deficiency in such receptors. The ionic time and voltage dependences for the anomalous rectifier in dorsal root ganglion neurons has been described, using voltage clamp techniques.